THE INTERNAL CONFIGURATION OF PLANET MERCURY. J. S.

46th Lunar and Planetary Science Conference (2015)
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THE INTERNAL CONFIGURATION OF PLANET MERCURY. J. S. Knibbe1 and W. van Westrenen1,
1
Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the
Netherlands, j.s.knibbe@vu.nl.
Introduction: The determination of Mercury’s
moment of inertia (C/MR2) of 0.346 ± 0.014 and the
fraction of polar moment induced by the outer solid
shell (Cm/C) of 0.431 ± 0.025 ([2] and [4]) has enabled
modelling studies to constrain the location of the coremantle boundary (CMB) to 2020 ± 70 km in radius ([1]
and [3]) assuming a core composition of binary ironsulfur or iron-silicon alloys, possibly overlain by a solid FeS-layer. The present-day radius of the inner core,
however, is poorly constrained. In this study we reexamine the implications of proposed interior configurations for Mercury and attempt to constrain the present-day inner core radius using recently improved
estimates for Mercury’s planetary contraction of up to
7 km in radius [5], assumed to be the consequence of
planetary cooling and, in particular, core solidification.
Method: A large set of density profiles for
Mercury have been generated in a Monte-Carlo approach similar to Hauck et al. [1] corresponding to the
interior configurations describe above with the mantle’s average density, light element content of the core,
CMB radius, inner core boundary (ICB) radius and
CMB temperature as varying parameters. For each of
these profiles, the planetary contraction due to core
solidification (ΔRcs) up to the ICB is calculated based
on the density difference between molten and solid
core material according to their respective equations of
state. Generated profiles are constrained by Mercury’s
total mass, the known C/MR2 and Cm/C and an upper
limit of 7 km for planetary radius contraction (ΔRcs).
Results: The CMB radius is constrained to
2005 ± 60 km for configurations without a solid FeSlayer. With an FeS-layer, the outer liquid core boundary (OLB) is located at 2004 ± 40 km, with a CMB radius ranging up to a maximum value of 2162 km. The
pressure at the CMB, or OLB for models with a solid
FeS-layer, is around 5.25 ± 0.3 GPa without an FeSlayer and around 5.5 ± 0.5 GPa with a FeS-layer. The
CMB radius correlates strongly with light element content of the core and the average density of the outer
solid shell. In general, a higher average mantle density
requires more light elements to accompany iron in the
core. The ICB significantly affects C/MR2 and Cm/C
only in the iron- sulfur core models, because sulfur’s
preferential partitioning into liquids generates a large
density jump at this boundary, whereas silicon’s nonpreferential liquid/solid partitioning doesn’t. Variations
in CMB temperature, that account solely for thermal
expansion in our model analysis, are unconstrained.
Figure 1: The residual squared error (ERSS) with respect
to the known C/MR2 and Cm/C ([2]), plotted against the
CMB radius for the generated density profiles of the
binary inro-silicon (a) and the iron-sulfur (b) core alloys without an FeS-layer. The colorbar denotes the
light element content in the core. The solid and dashed
lines denote ERSS = 1 and ERSS = 2, respectively.
However, relating liquid-solid phase equilibria at the
ICB adiabatically to the CMB temperature suggests
that a temperature of 2050 °C at the CMB represents
an upper temperature limit for the occurrence of a pure
iron solid inner core. Cores of iron-silicon and ironsulfur alloy yield a lower limit for the CMB temperature of 1700 °C and 1400 °C, respectively, to maintain
liquid material in the outer core. Furthermore, in a binary iron-sulfur core without a solid FeS-layer the sulfur content is constrained to values lower than 8 wt%
by C/MR2, Cm/C and planetary contraction, whereas
core silicon contents are unconstrained within the range
examined (0-17wt%). If the core is overlain by a significant FeS-layer, high core abundances of light elements
are required in the rest of the core to compensate for
the increased mass of the planet’s solid outer shell (that
includes the FeS-layer).
46th Lunar and Planetary Science Conference (2015)
Figure 2: Radial planetary contraction plotted against
the radius of the present day inner core (a and b) or the
width of the solid FeS-layer (c). The color bar denotes
the light element content in the core (a and b), or the
pressure of the outer liquid boundary of the core (c).
Large dots and squares denote profiles with ERSS < 2
and < 1, respectively (figure 1). The miniplot in (b) is a
zoomed in version of the larger plot in (b).
The calculated ΔRcs values constrain the ICB
radius to below 1800 km for a pure iron core. Increasing the silicon and sulfur contents up to 17 wt% and 8
wt% respectively lowers this upper bound to 1650 km
and 1400 km in radius respectively. For models with a
solid FeS-layer on top, ΔRcs estimates relate linearly to
the width of the layer with a slope of around 0.025
contraction (km) per width of the FeS layer (km).
Discussion: The results broadly confirm findings of [1] and [3]. However, the interrelations between
the free variables are more thoroughly examined than
in [1] and more core alloys are examined than only the
iron-sulfur core in [3]. In this respect, the
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Figure 3: Average core density plotted against the average mantle density. The colorbar denotes the light
element content in a core of a binary iron-silicon (a) or
iron-sulfur alloy (b). Large dots and squares denote
profiles with ERSS < 2 and < 1, respectively (figure 1).
results provide a more detailed and broader insight of
Mercury’s interior configuration. The ICB radius, here
constrained by ΔRcs < 7 km, is still poorly constrained
for configurations without an solid FeS-layer and low S
contents. However, because the presence of an FeSlayer requires higher light element contents for the rest
of the core, not only does the solidification of an FeSlayer directly induces planetary contraction, the ΔRcs
estimates additionally increase due to the high abundance of light core elements in the rest of the core. For
this reason, the likelihood that both a large inner core
and an FeS-layer of significant width are present is
low.
References
[1] Hauck S. A. et al. (2013), JGR-Planets
118, 1204-1220. [2] Margot J. L. et al. (2012), JGR,
117. [3] Rivoldini A. and van Hoolst T. (2013), EPSL,
377-378, 62-72. [4] Smith D. E. et al. (2012), Science
336, 214-217. [5] Byrne P. K. et al. (2014), NatureGeosci 7, 301-307.